Title

Author

Date of Award

2000

Availability

Article

Degree Name

Doctor of Philosophy (Ph.D.)

First Committee Member

Narendra K. Simha, Committee Chair

Abstract

This thesis discusses the fatigue and fracture properties of NiTi polycrystals. Systematic experiments based on a functional phase diagram in the space of mean strain and strain amplitude show that NiTi has novel fatigue properties. The cycles-to-failure has a non-monotone dependence on the mean strain at fixed strain amplitude. Analysis of stress-strain data and SEM fractographs indicates that the stress concentrations at the edges of a macroscopic phase boundary may be responsible for the drastic decrease in the cycles-to-failure in NiTi specimens containing both austenite and martensite.A further set of experiments were designed to establish a candidate region for the infinite life of NiTi and to analyze the effects of mean strain, strain amplitude, surface treatment and some additional test variables on the fatigue life. The experiments were designed using the same functional phase diagram and additionally an 8-parameter ruggedness test scheme was implied. The results are successful in terms of establishing a candidate space for infinite life of NiTi in mean strain-strain amplitude space. The effects of surface treatments did not reveal distinct effects of surface treatments since failure did not occur in most of the tested specimen.Next, the result of fracture test on NiTi sheet specimen is reported. The test was performed using ASTM J integral testing guidelines. The results are analyzed by interpreting the strain energy release rate as the J integral value. The results show that the high stress intensity factor at the crack tip causes the material surrounding the crack tip to transform to martensite. The strain energy is dissipated by the crack growth and the propagation of the phase front.